Ammonium paratungstate hydrates and ammonium paratungstate decahydrate

ABSTRACT

The invention is directed to ammonium paratungstate decahydrate containing at least 75% of crystals having a length of at least 200 μm and having a ratio of length to width of less than 4.5:1.

RELATED APPLICATIONS

This Application is a Divisional Application of U.S. application Ser.No. 12/524,957 filed on Aug. 19, 2009 which is incorporated byreference. U.S. application Ser. No. 12/524,957 is a national stageapplication (under U.S.C. §371) of PCT/EP2008/050995, filed Jan. 29,2008, which claims benefit of German application 10 2007 005 286.5,filed Feb. 2, 2007.

BACKGROUND OF THE INVENTION

The present invention relates to a novel process for preparing highlypure ammonium paratungstate hydrates and ammonium paratungstatedecahydrate having a selected crystal structure.

Ammonium paratungstate hydrates (hereinafter referred to as APTs) areknown intermediates for producing tungsten metal, tungsten-containingcatalysts or hard materials based on tungsten, for example tungstencarbides.

The preparation of highly pure APT is carried out essentially via acidor alkali digestion or fusion of tungsten-containing concentrates ortungsten scrap with subsequent purification stages involvingprecipitation processes and liquid-liquid extraction. The purifiedsolution is generally concentrated by evaporation, resulting in APTfinally crystallizing out. The publication “KristallographischeUntersuchung der Ammoniumparawolframate (Zeitschrift fürKristallographie, vol. 120, pp. 216-228 (1964))” by M. Hähnert statesthat APT×10H₂O is formed on slow evaporation of an ammonium tungstatesolution prepared from WO₃ and NH₃ solution. The crystals have anacicular morphology. The bulk density of the product is 0.7 g/cm³. Thisis not an industrial process but a preparative laboratory process.

Further routes to the preparation of APT×10H₂O are described in thearticle “Characterisation of various commercial forms of ammoniumparatungstate powder, Journal of Material Science, No. 10 (1975), pages571-577”.

In one process variant, an ammonium tungstate solution prepared fromtungstic acid and an excess of NH₃ solution is subject to freeze drying.The product formed has a crumbly poorly defined crystal morphology andthe bulk density is 1.03 g/cm³. In a second process variant, an ammoniumtungstate solution prepared as described above is slowly neutralizedwith HCl solution at room temperature. The acicular crystals formed havea length of 16-70 μm and a width of 2-14 μm and the bulk density is 1.46g/cm³. Both processes form APT×10H₂O having a low bulk density and thenecessary product purity can be achieved only when using clean tungsticacid. In addition, a freeze drying procedure in one case and the use ofclean HCl solution in the other case leads to very high process costs.When HCl is used, the product is contaminated with chloride as a resultof the process.

In the specialist book “Metallurgie der seltenen Metalle, Seligman,Krejn and Samsonov (1978), Metallurgia publishers (USSR), pp. 62-63”, anindustrial process for preparing APT×10H₂O is described as follows.Scheelite concentrate is digested with HCl solution at 90-100° C. so asto form tungstic acid. The tungstic acid is subsequently dissolved inNH₃ solution and the solution is cooled. The resulting ammoniumtungstate solution is slowly neutralized with HCl solution to a pH of7.3-7.4 while stirring. After allowing to stand for 24 hours, theAPT×10H₂O product is separated off. The acicular crystals have a lengthof 15-25 μm and a width of 1-3 μm and the bulk density is 0.98 g/cm³.The crystallization yield here is 85-90%. This product still containsconsiderable amounts of impurities. A further disadvantage of thisprocess is the high consumption of clean HCl solution and the largeamount of W-containing NH₄Cl solution (mother liquor) which has to beworked up.

FIG. 1 depicts a scanning electron micrograph (SEM) of a productprepared by this process.

It is known that clean ammonium tungstate solutions are used forpreparing highly pure W salts. These solutions are usually preparedindustrially by sodium hydroxide or sodium carbonate fusion of Wconcentrates and subsequent precipitation of P, As, Si and Mo impuritiesby addition of Mg, Al salts and sodium hydrogensulfide and then carryingout a liquid-liquid extraction using amine-containing organic phases.The parasitic formation of APT×10H₂O when a liquid-liquid reextractionwith NH₃ solution is carried out is mentioned in U.S. Pat. No. 4,450,144and U.S. Pat. No. 4,092,400. However, the aim of these processes is toprepare clean ammonium tungstate solution which can be converted byevaporative crystallization into APT×4H₂O. The formation of APT×10H₂O inthe reextraction in the processes mentioned has an adverse effect on thephase separation, the purity of the APT×4H₂O product and thecrystallization yield. For this reason, the abovementioned patent textsdescribe possible ways of reducing or preventing the formation ofAPT×10H₂O crystallites in the reextraction.

A process for preparing APT via digestion of W-containing concentrateswith subsequent liquid-liquid extraction of the tungsten compounds andsubsequent reextraction with NH₃ solution is described inDE-B-1,150,962. Here too, an organic amine phase (tertiary alkylamine)is used for separating tungsten from W-containing digestion solution.According to this process, as can be seen from the accompanying example,the organic amine phase laden with 23-27 g/l of tungsten is placed in asettling apparatus in the form of a long tower and reextracted bydropwise addition of 5-29% strength NH₃ solution. The reextraction iscarried out at an NH₃:W molar ratio of the starting solutions in therange from 3.6:1 to 50.1:1, depending on the embodiment, and a ratio oforganic phase (OP) to aqueous NH₃ solution in the range from 2.1:1 to5.5:1. The APT product formed is subsequently filtered off and dried.Carrying out the reextraction by this process leads to a finelycrystalline acicular APT×10H₂O product having OP adhering to the surfaceand a low bulk density of <1.0 g/cm³. FIG. 2 depicts a scanning electronmicrograph of a product prepared by this process. Chemical analysis ofthe product shows a high proportion of carbon contamination of 5000-10000 ppm. For these reasons, the material is not suitable for furtherprocessing steps. In addition, the phase separation in the reextractionas described in DE-B-1,150,962 occurs only after long standing. This canbe attributed to the finely crystalline character of the product. Owingto the high W content of the mother liquor, which is due to the NH₃:Wmolar ratio used in the reextraction, the crystallization yield in thisprocess is not more than 65% (see abovementioned publication, experiment4). The poor product quality, the poor phase separation and the lowcrystallization yield has resulted in this process not having beenimplemented to the present time.

For these reasons, the further developments of W reextraction fromamine-containing organic phases have gone in the direction ofliquid-liquid reextraction with avoidance of APT precipitation andsubsequent APT production by evaporation of the clean ammonium tungstateextract solutions, as described in the abovementioned documents U.S.Pat. No. 4,450,144 and U.S. Pat. No. 4,092,400.

BRIEF SUMMARY OF THE INVENTION

The invention relates to a process for the continuous preparation ofammonium paratungstate hydrate directly in the reextraction of atungsten-laden organic phase with an ammonia-containing aqueous solutionin a mixer-settler apparatus, which comprises carrying out thereextraction at an NH3:W molar ratio of from 0.83 to 1.30, and a volumefeed ratio of the tungsten-laden organic phase to the ammonia-containingaqueous solution of from 5 to 25. The invention is also directed toammonium paratungstate decahydrate containing at least 75% of crystalshaving a length of at least 200 μm and having a ratio of length to widthof less than 4.5:1.

In the light of the prior art, it is an object of the present inventionto provide a comparatively cheap and simple process which allows highlypure coarsely particulate ammonium paratungstate hydrate to be preparedwith a high crystallization yield in a continuous process.

A further object of the present invention is to provide a process forpreparing coarsely particulate ammonium paratungstate hydrate, in whichthe product crystallizes directly during the reextraction.

A still further object of the present invention is to provide a highlypure ammonium paratungstate decahydrate having a selected crystalstructure and a high bulk density.

A BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a finely crystalline ammonium paratungstatedecahydrate prepared as described in “Metallurgie der seltenen Metalle,Seligman, Krejn and Samsonov (1978), Metallurgia publishers (USSR), pp.62-63”.

FIG. 2 illustrates a finely crystalline ammonium paratungstatedecahydrate prepared as described in DE B 1,150,962.

FIG. 3 illustrates an embodiment of the process of the invention forpreparing APT.

FIG. 4 illustrates an APT decahydrate prepared by the process of theinvention (scanning electron micrograph (SEM)).

FIG. 5 illustrates an APT decahydrate prepared by the process of theinvention (X ray diffraction pattern (XRD)).

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present description, the term ammoniumparatungstate hydrate encompasses the tetrahydrate, i.e.(NH₄)10[H₂W₁₂O₄₂]×4(H₂O), and the decahydrate, i.e.(NH₄)10[H₂W₁₂O₄₂]×10(H₂O). The formation of these hydrates occurs as afunction of the reextraction temperature.

No process in which coarsely crystalline APT can be crystallizeddirectly during the reextraction has hitherto been described in theprior art. The known processes give reextraction solutions which have ahigh NH₃:W ratio and have to be concentrated by energy-consuming methodsand freed of the excess of ammonia or require other measures for settingthe necessary pH, e.g. by acidification with a mineral acid. In anotherprocess, APT is crystallized directly during the reextraction, but theproduct is very finely crystalline and difficult to free of organicimpurities or has only a low purity. In addition, the resultingcrystallization yield in this process is very low.

We have now surprisingly found a process which allows the isolation ofcoarsely crystalline highly pure APT with a very high crystallizationyield directly in the reextraction.

Apart from the saving of intermediate steps and the associated saving ofenergy and apparatuses, the process of the invention has furtheradvantages. It can be carried out in a simple manner and the amount ofammonia to be used is considerably lower than in known processes.

The present invention provides a process for the continuous preparationof APT directly in the reextraction of a tungsten-laden organic phase(OP) with an ammonia-containing aqueous solution (AP) in a mixer-settlerapparatus, characterized in that the reextraction is carried out at anNH₃:W molar ratio of from 0.83 to 1.30, preferably from 0.85 to 0.95,and an OP:AP volume feed ratio of the starting materials of from 5 to25, preferably from 10 to 15.

In a preferred embodiment, the reextraction in the mixer is carried outso that an APT solids concentration, based on the aqueous phase in themixer, of from 100 to 1200 g/l, preferably from 500 to 800 g/l, isestablished.

The stirring speed in the mixer is set so that the distribution oforganic phase and ammonia-containing aqueous phase is not homogeneous.

The OP and AP are generally introduced in the lower half of the mixer,preferably in the region of the stirrer, and the three-phase mixtureformed is taken off from the upper region, preferably via a freeoverflow, and from the lower region, preferably from the region of thestirrer, so that a steady-state OP:AP phase ratio in the range from 1:5to 1:70 is established in the mixer.

The phase separation is preferably carried out in a downstream settlerapparatus equipped with stirrer while stirring slowly, with therotational speed of the stirring device being set so that entrained OPis separated off from the APT phase during sedimentation of the APT. Thephase mixture from the mixer is preferably introduced into the OP-APphase boundary region of the settler.

A proportion of the aqueous phase formed after the phase separation(mother liquor) is preferably recirculated to the mixer so that an APTsolids concentration of 100-1200 g/l, preferably 500-800 g/l, isestablished in the mixer.

The feed rates of the starting solutions for the reextraction arepreferably selected so that an OP residence time in the mixer of atleast one minute and an AP residence time of more than 3 hours areestablished, with the OP residence time being 60 times the ratio of thevolume of the OP in the mixer in liters to the feed rate of the OP inliters/hour and the AP residence time being the ratio of the volume ofthe AP in the mixer in liters to the feed rate of the AP in liters/hour.

The idea of the preferred embodiment of the process of the invention isto set a phase distribution between organic and aqueous phases which isinhomogeneous over the height of the mixer in the reextraction of atungsten-laden organic phase, to set a selected feed ratio of thestarting materials and a selected molar ratio of ammonia and tungsten inthe streams fed to the extraction apparatus and to ensure a selectedresidence time of the aqueous and organic phases in the extractionapparatus.

It has surprisingly been found that carrying out the reextraction usinga low NH₃:W molar ratio, a high proportion of aqueous phase in thesteady-state phase mixture in the mixer with an inhomogeneous phasedistribution and a high APT solids content leads to a highly puregranular APT product.

It is also surprising that combined taking-off of the phase mixture fromthe upper third and from the region of the stirrer in the mixer atappropriately chosen ratios of the two discharge streams and incombination with the rotational speed of the stirrer enables therequired steady-state phase ratio in the mixer to be set independentlyof the feed ratio of the OP/AP starting solutions.

The process of the invention is preferably carried out in a mixerapparatus which is equipped with an adjustable stirrer and combinedtaking off of product from the upper region, preferably via a freeoverflow, and from the lower region, preferably from the region of thestirrer, and a settler apparatus which is equipped with a slow-runningstirrer.

A preferred embodiment for carrying out the process of the invention isshown in FIG. 3.

It has been found that all the measures in the process of the inventionwhich are described in claim 1 are essential for achieving the intendedpurpose and that the measures in the process of the invention which aredescribed in the dependent claims are particularly advantageous forachieving the intended purpose.

Thus, an NH₃:W molar ratio of <0.83 leads not only to a deterioration inthe reextraction yield and phase separation but to formation of depositson plant components contacted by the OP. An NH₃:W molar ratio of >1.30leads to a reduction in the crystallization yield and a deterioration inthe product purity.

A steady-state ratio in the mixer of OP:AP of >1:5 leads to formation ofa finely crystalline acicular product having a very high proportion ofcarbon contamination and to a deterioration in the phase separation. Asteady-state OP:AP ratio in the mixer of <1:70, on the other hand, leadsto a reduction in the reextraction yield and to formation of deposits onplant components contacted by the OP.

Setting of an OP/AP phase feed ratio of the starting materials of <5:1leads to formation of a metastable aqueous phase which leads tocrystallization of APT on the walls of the apparatuses/pipes. A phaseratio of >25:1, on the other hand, leads to a drastic increase inimpurity contents in the product.

In the case of a homogeneous phase distribution in the mixer, it is notpossible to set the steady-state phase ratio in the mixer independentlyof the phase feed ratio of the starting materials.

The type of APT formed is dependent on the reextraction temperature. Attemperatures up to 60° C., the decahydrate is formed, while attemperatures above 60° C., the tetrahydrate is formed. The preferredtemperature for the preparation of decahydrate is from 45 to 55° C. andthat for the preparation of tetrahydrate is from 80 to 98° C.

In a preferred variant of the process of the invention, a proportion ofthe aqueous phase (mother liquor) formed after phase separation in thesettler is recirculated to the mixer, so that an APT solidsconcentration in the mixer of from 100 to 1200 g/l, preferably from 500to 800 g/l, is established.

A steady-state solids content in the vessel of <100 g/l leads toformation of deposits on the walls of the apparatuses/pipes. Asteady-state solids content in the vessel of >1200 g/l, on the otherhand, leads to an increase in carbon contamination in the product, to adeterioration of phase separation and to finely crystalline endproducts.

In a likewise preferred variant of the process of the invention, thestarting solution and the ammonia-containing aqueous solution areintroduced into the stirrer region of the mixer. Introduction of thestarting solution and the ammonia-containing aqueous solution(s) intothe stirrer region leads to an improvement in the reextraction yield andto a decrease in deposits on the walls of the apparatuses/pipes.

In a further preferred variant of the process of the invention, the feedrates of the starting solutions to the reextraction are selected so thatan OP residence time in the mixer of at least one minute and an APresidence time of more than three hours are established, with the OPresidence time being 60 times the ratio of the volume of the OP in themixer in liters to the feed rate of the OP in liters/hour and the APresidence time is the ratio of the volume of the AP in the mixer inliters to the feed rate of the AP in liters/hour.

An OP residence time in the mixer of less than 1 minute leads to areduction in the reextraction yield and to formation of deposits onplant components which come into contact with OP. An OP residence timein the vessel of more than 10 minutes is unfavorable because of thedeterioration in the space-time yield.

An AP residence time in the mixer of less than 3 hours leads to a finelycrystalline product, to a reduction in the crystallization yield and toformation of deposits on the walls of the apparatuses/pipes. An APresidence time in the mixer of more than 10 hours, on the other hand,leads, at comparable product properties and crystallization yields, to areduction in the space-time yield.

The phase mixture from the mixer is advantageously introduced into thesettler in the vicinity of the OP/AP boundary, so that entrainment of OPin the sedimentation of APT is drastically reduced. In addition, phaseseparation is carried out with slow stirring, so that OP entrained bythe product is separated off.

Furthermore, separation of the phase mixture with slow stirring leads toa significant reduction in the separation time and thus to an increasein the space-time yield in the settler.

The organic phase comprises, for example, 7% by weight ofdiisotridecylamine, from 10 to 15% by weight of isodecanol and 78-83% byweight of an aliphatic hydrocarbon mixture (e.g. petroleum spirit K₆₀)and is laden in a manner known to those skilled in the art with from 40to 80 g/l of tungsten, preferably from 60 to 70 g/l. Apart fromsecondary amines, it is also possible to use tertiary amines orquaternary ammonium salts and other modifiers instead of isodecanol andalso other hydrocarbon mixtures, including those having othercomposition ratios.

The invention also provides a novel ammonium paratungstate decahydratewhich comprises at least 75% of crystals having a length of at least 200μm and having a ratio of length to width of <4.5:1. Such a product canbe prepared by the above-described process. In contrast to previouslyknown products, this product is significantly coarser, has smalleramounts of impurities and can be more readily processed further.

The ammonium paratungstate decahydrate of the invention preferably has abulk density of at least 1.7 g/cm³, in particular from 1.8 to 2.2 g/cm³.Such bulk densities could not hitherto be obtained for this product.

The bulk density was determined in accordance with ASTM B329.

The ammonium paratungstate decahydrate preferably has a length of from200 to 1000 μm, particularly preferably from 300 to 400 μm.

The ammonium paratungstate decahydrate likewise preferably has a lengthof from 300 to 400 μm and a ratio of length to width of from 3.0:1 to3.5:1.

This ammonium paratungstate decahydrate is particularly preferred ashigh-purity product, for example characterized by a purity of at least99.99%, based on the total mass of the product.

The invention is described in FIG. 4 and the following example.Restriction to this example and this figure is not implied thereby.

The figures show:

FIG. 1: a finely crystalline ammonium paratungstate decahydrate preparedas described in “Metallurgie der seltenen Metalle, Seligman, Krejn andSamsonov (1978), Metallurgia publishers (USSR), pp. 62-63”.

FIG. 2: a finely crystalline ammonium paratungstate decahydrate preparedas described in DE-B-1,150,962.

FIG. 3: an embodiment of the process of the invention for preparing APT.

FIG. 4: an APT decahydrate prepared by the process of the invention(scanning electron micrograph (SEM)).

FIG. 5: an APT decahydrate prepared by the process of the invention(X-ray diffraction pattern (XRD)).

Example

W concentrate was digested by means of sodium hydroxide and the liquorformed was subjected to preliminary removal of impurities such as P, As,Si, V and Mo by addition of Mg salts, Al salts and sodiumhydrogen-sulfide. As further purification step to remove anionic andcationic impurities still present, a liquid-liquid extraction using anorganic phase (7-10% by weight of diisotridecylamine, 10% by weight ofisodecanol, balance: petroleum spirit) was carried out. The OP ladenwith tungsten was reextracted using NH₃ solution. The apparatus used forthis purpose is shown in FIG. 3.

400 l/h of W-laden OP and a regulated amount of NH₃ solution were fedcontinuously into the stirrer region of a stirred vessel (1) equippedwith stirrer (2) and baffles (3) (also referred to as mixer) (volume:250 l, diameter: 600 mm, inclined-blade stirrer: 6 blades, diameter: 300mm, 4 baffles) via lines A, B and C. Water was introduced via line B.The W concentration of the OP and the NH₃ concentration of the NH₃solution was measured automatically in-line. The metering of the NH₃solution was regulated automatically via the NH₃:W molar ratio set at0.90. The OP/(NH₃ solution+water) feed ratio was set to 15:1 andlikewise regulated automatically via the water flow (B) into the NH₃feed line (A).

The volume flow of the OP (average W loading: 62.0 g/l) was set to afixed value. The volume flow of the NH₃ solution was regulated as afunction of the volume flow of the OP, the instantaneously measured Wand NH₃ concentrations and the NH₃:W molar ratio set. The H₂O volumeflow was regulated as a function of the volume flow of the NH₃ solutionand the feed ratio of the starting materials set. The temperature in themixer (1) was set to 50° C. and regulated via temperature control of thefeed solutions.

The transfer of the 3-phase mixture formed in the mixer (1) to thesettler (6) was effected from the stirrer region of the vessel via theoutlet (5) and line D and also via the free overflow (4) of the vesseland line E.

The steady-state OP/AP ratio of 1/8 in the mixer (1) and thesteady-state solids concentration of 750 g/l based on the aqueous phase(NH₃ solution+mother liquor+water) was set via the rotational speed ofthe stirrer (210 rpm), the offtake of phase mixture from the lowerregion of the mixer (50 l/h) and the recirculation of aqueous phase(mother liquor) from the settler (6) to the mixer (1) (20 l/h). Theresidence time of the AP in the mixer (1) was 4.8 hours based on thesteady-state proportion of AP in the mixer and that of the OP was 4.2minutes based on the steady-state proportion of OP in the mixer. Thephase mixture was separated in the settler (6) equipped with aslow-running stirrer (7) (volume: 600 l, diameter: 750 mm (shape:conically tapering above half the height; equipped with a slow-runningstirrer of the anchor type running around the wall (tapered obliquely)).The rotational speed of the slow-running stirrer was set to 15 rpm. Thestripped OP was separated off via the overflow of the settler via lineF, washed with water and recirculated to the loading stage of theliquid-liquid extraction. The product suspension having an APT solidscontent of 1314 g/l was transferred from the lower region of the settler(6) via line G into an intermediate vessel (8) equipped with stirrer (9)and baffles (10) as buffer vessel prior to filtration. From there, theaqueous APT suspension was transferred via line H into the filter (11)and the APT×10H₂O was washed with a little water in order to displacethe mother liquor. The product was taken from the filter and finallydried at 50° C. and characterized.

The mother liquor containing 35 g/l of W and 7.0 g/l of NH₃ wasdischarged from the filter (11) via line J and, after removal of theNH₃, added to the digestion solution for the W concentrates. Thecrystallization yield based on the W content of the OP is 96.2%.

The APT×10H₂O obtained as described in this example has a very highpurity of >99.99%. The analyses of different daily batches of thisproduct are shown in the following table. FIG. 4 shows a scanningelectron micrograph of the product prepared in the example.

TABLE Chemical analysis and bulk density of various daily batches of theproduct prepared as described in the example. Bulk W NH₃ F C P Si Al CaCr Fe K Li Mo Na V density Sample [%] [%] [ppm] [ppm] [ppm] [ppm] [ppm][ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [g/cm³] 1 68.36 5.23 <1018 <1 <2 <3 <3 <2 <2 <1 <1 <3 <1 <1 1.98 2 68.63 5.24 <10 29 <1 <2 <3 <3<2 <2 <1 <1 <3 <1 <1 2.12 3 68.67 5.27 <10 23 <1 <2 <3 <3 <2 <2 <1 <1 <3<1 <1 1.79 The associated X-ray diffraction patterns (XRD) of the threesamples are depicted in FIGS. 5a to 5c.

The invention claimed is:
 1. An ammonium paratungstate decahydrate comprising at least 75% of crystals having a length of at least 200 μm and having a ratio of length to width of less than 4.5:1.
 2. The ammonium paratungstate decahydrate as claimed in claim 1, wherein the ammonium paratungstate decahydrate has a bulk density of at least 1.7 g/cm³.
 3. The ammonium paratungstate decahydrate as claimed in claim 1, wherein the ammonium paratungstate decahydrate has a bulk density of from 1.8 to 2.2 g/cm³.
 4. The ammonium paratungstate decahydrate as claimed in claim 1, wherein at least 75% of the crystals have a length of from 200 to 1000 μm.
 5. The ammonium paratungstate decahydrate as claimed in claim 1, wherein at least 75% of the crystals have a length of from 300 to 400 μm.
 6. The ammonium paratungstate decahydrate as claimed in claim 1, wherein at least 75% of the crystals have a length of from 300 to 400 μm and a ratio of length to width of from 3.0:1 to 3.5:1.
 7. The ammonium paratungstate decahydrate as claimed in claim 3, wherein at least 75% of the crystals have a length of from 300 to 400 μm and a ratio of length to width of from 3.0:1 to 3.5:1.
 8. The ammonium paratungstate decahydrate as claimed in claim 1, wherein the ammonium paratungstate decahydrate has a purity of at least 99.99%, based on the total mass of the product.
 9. The ammonium paratungstate decahydrate as claimed in claim 7, wherein the ammonium paratungstate decahydrate has a purity of at least 99.99%, based on the total mass of the product. 